Silver halide photothermographic imaging materials, often referred to as "dry silver" compositions because no liquid development is necessary to produce the final image, have been known in the art for many years. These imaging materials basically comprise a light insensitive, reducible silver source, a light sensitive material which generates silver when irradiated, and a reducing agent for the silver source. The light sensitive material is generally photographic silver halide which must be in catalytic proximity to the light insensitive silver source. Catalytic proximity is an intimate physical association of these two materials so that when silver specks or neclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the silver source by the reducing agent. It has been long understood that silver is a catalyst for the reduction of silver ions and the silver-generating light sensitive silver halide catalyst progenitor may be placed into catalytic proximity with the silver source in a number of different fashions, such as partial metathesis of the silver source with a halogen-containing source (e.g., U.S. Pat. No. 3,457,075), coprecipitation of the silver halide and silver source material (e.g., U.S. Pat. No. 3,839,049), and any other method which intimately associates the silver halide and the silver source.
Photothermographic emulsions tend to suffer from post development print stability when the Dmin areas are exposed to the high intensity light and heat from viewboxes. Traditional photothermographic materials have suffered from print stability. The minimum density areas darken when samples are left on viewboxes where the combination of light and heat tend to darken the background density. Adding to the difficulty of print stability is the fact that the developer, toners, and silver are incorporated in the photothermographic element which is not the case in most silver halide photographic systems. Likewise the light and heat from the viewbox are mere extensions of the light and heat used in the imaging of the sheet. The need for improved print stability is therefore always considered to be very important.
Many attempts have been made to improve the post development print stability of the photothermographic element. U.S. Pat. No. 4,012,260 describes improvements by adding 2-amino-2-thiazolinium carboxylates. U.S. Pat. No. 3,877,940 uses a precursor combination of a blocked thione and a halogen-containing stabilizer. U.S. Pat. No. 3,707,377 incorporates tribromoquinaldines and hexabromocyclohexane to suppress background discolouration. The addition of an image stabilizer precursor comprising 5-acetyl-4-methyl-2-(3-oxobuty-lthio) thiazole is used in U.S. Pat. No. 3,839,041. Oriental Photo Ind. Co. Ltd. in JP 0288039 stabilized the thermal sheet by adding tribromoacetophenone on a polyacrylic resin substrate to produce the stabilized sheet.
Tetrabromobutane is described as a post development print stabilizer in U.S. Pat. No. 4,108,665 and U.S. Pat. No. 4,288,536. Post-processing stabilization using amido compounds is described in EP 473 35 1 A2. The post-processing stability of silver halide photothermographic emulsions is enhanced with certain azlactones in EP 480 568 AZ. In U.S. Pat. No. 5,149,620 post-processing stability is improved by the addition of mercapto triazoles. However, these compounds were not found to produce sufficient post-development print stability on the photothermographic element for use in a view box.